The present invention relates to a rolling bearing, and more particularly, a needle bearing to aid in power transmission within a transmission, or the like, used for automobiles, agricultural machinery, construction equipment, etc. More particularly, the present invention relates to a rolling bearing such as a comparatively small sized precision ball bearing used for supporting a rotating spindle which is incorporated in a fan motor for air-conditioning purposes, a cooling fan motor used in equipment of various types, a hard disk drive (hereinafter abbreviated as HDD), or a video tape recorder (hereinafter abbreviated as VTR).
In an automobile, or the like, a rolling bearing is used for a rotational section of a transmission which transmits power. A needle bearing is used in many cases in terms of load capacity or space. Particularly, needle bearings including radial needle bearings and thrust needle bearings are heavily used for automatic transmission.
High carbon chrome bearing steel 2 (SUJ2) has been used as material for the rolling bearing of this type and is composed of 0.95 to 1.05 wt. % C, 0.15 to 0.35 wt. % Si, 0.5 wt. % or less Mn, 1.30 to 1.60 wt. % Cr, with the remainder being Fe and inevitable impurities.
The dimensional accuracy, such as roundness, of coiled material used for a rolling element of the rolling bearing must be strictly set. For this reason, as shown in FIG. 1, after having undergone a softening process 52, a hot-rolled coil 51 is usually subjected to a round of processes repeatedly: namely, a pickling process 53, a coating process 54, and a cold drawing process 55, to thereby manufacture high-precision coiled material 56.
In recent years, another technique has been developed by virtue of an improvement in rolling techniques. As shown in FIG. 2, coiled material having the degree of precision as that of solid-drawn material is manufactured by subjecting the hot-rolled coil 51 to a precision cold rolling process 57. Subsequently, as by the manufacturing method shown in FIG. 1, the coiled material is subjected to a round of processes: i.e., the softening process 52, the pickling process 53, the coating process 54, and the cold drawing process 55, to thereby manufacture the high-precision coiled material 56 (this conventional technique will be hereinafter referred to as the "first conventional technique").
There is a known technique intended to obtain a rolling bearing which has superior durability and is prevented from sustaining so-called peeling damage or sliding wearing. According to this technique, an element having superior wearing resistance, such as Cr, Mo, or V, is added to the material of a bearing ring or a rolling element. Alternatively, the material of a bearing ring or a rolling element is subjected to a carburizing or carbonitriding treatment to thereby increase the surface hardness or compression residual stress of the bearing. Still another technique is proposed, wherein a plurality of concave indentations are formed in the surface of a bearing ring or rolling element from analogous viewpoints to thereby control the ratio of axial surface hardness to circumferential surface hardness (as described in Japanese Patent Application Unexamined Publication Nos. Hei-2-168021 and Hei-3-117724) (these conventional techniques will be hereinafter referred to as the "second conventional techniques").
A comparatively small precision ball bearing (hereinafter simply referred to as a "ball bearing") used for a fan motor for air conditioning purposes or used for supporting a rotating spindle of a HDD is used under comparatively light load. However, there is a demand that vibrations or noise produced from the bearing be reduced. In some cases, the ball bearing reaches its limit of use by reason of acoustic deterioration. Because of this, the ball bearing is desired to have superior acoustic properties so that the ball bearing can be prevented from reaching its limit of use as a result of acoustic deterioration before reaching its fatigue strength.
From the foregoing viewpoints, several techniques are proposed: e.g., a technique of reducing the amount of residual austenite which is included in steel and is considered to deteriorate the acoustic characteristics of the ball bearing to a certain amount (as described in Japanese Patent Application Unexamined Publication No. Hei-7-103241), or a technique of increasing the surface hardness of a ball bearing so as to have superior resistance to acoustic deterioration by subjecting at least the material of a bearing ring to a carbonitriding treatment or increasing the percentage content of Si included in a bearing ring to thereby reduce the amount of residual austenite to zero (as described in Japanese Patent Application Unexamined Publication No. Hei-8-312651) (these conventional techniques will be hereinafter referred to as the "third conventional techniques).
The first conventional technique enables a high-precision coil material which can be used as material for a rolling element of a bearing to be obtained at a low cost through one cold drawing operation. However, if lubrication conditions are inappropriate or foreign articles are mixed into a bearing, an inner-outer race serving as a bearing ring comes into metal-to-metal contact with a rolling element because of an insufficient layer of oil while the bearing is in use. Consequently, the bearing is susceptible to surface damage such as peeling damage or sliding wearing. More specifically, in a transmission system of an automobile, the passage of lubrication of a bearing is complicated from structural viewpoints. Because of this, a lubricant is sometimes insufficiently fed to the inside of the bearing. Thus, there are many cases where the bearing is used under the harsh lubrication condition. If the bearing is used under such harsh lubrication condition, the layer of oil is insufficiently formed in the area where the rolling element comes into contact with the bearing ring, bringing them into direct metal-to-metal contact with each other. A so-called rolling surface sustains surface damage such as peeling damage or sliding wearing, resulting in the risk of early peeling or anomalous vibrations.
The second conventional techniques enable the bearing to be prevented from sustaining surface damage such as peeling damage or sliding wearing. However, these techniques require the control of surface roughness of a bearing by subjecting the bearing to special treatment after the bearing has undergone a grinding operation, inevitably adding to the cost.
There are considered various causes of vibrations or noise which develops in the ball bearing. In the foregoing comparatively small ball bearing, an increase in the vibrations or noise which develops in the raceway surface of the bearing ring is considered to be the principal cause of vibrations and noise. More specifically, the amount of residual austenite-which is permanently deformed under impact load, or the like, so that the raceway surface is susceptible to impressions-is considered to be the principal cause of acoustic deterioration. For this reason, a reduction in the amount of residual austenite or the removal of residual austenite is considered to be effective countermeasure against acoustic deterioration.
However, as in the foregoing third conventional techniques, the reduction in the amount of residual austenite or the removal of residual austenite involves a so-called subzero treatment or a high-temperature tempering treatment, thereby adding to the cost.
The conventional third techniques are intended to remove residual austenite, which is the cause of impression, to as small an amount as possible and are not intended to reduce the vibrations or noise originating from the raceway surface. Therefore, the acoustic characteristics of the ball bearing are not drastically improved by the third techniques.